Add new (unimplemented) TCK test

com.oracle.truffle.r.test/src/com/oracle/truffle/r/test/tck/FastRTckTest.java

@@ -164,6 +164,12 @@ public class FastRTckTest extends TruffleTCK {
         return "valuesObject";
     }
 
+    @Override
+    protected String addToArray() {
+        // TODO not yet supported
+        return null;
+    }
+
     @Override
     public void readWriteBooleanValue() throws Exception {
         // TODO not yet supported